In electrophotographic printing machines, an image is produced on a photoreceptive surface through the operation of a character generator. The character generator may be, for example, a light source which is driven from information held in a digital memory. The light source may be a laser gun, an array of light emitting diodes, light modulators, etc. which produce light rays which are directed to the photoreceptor and cause it to bear a charge pattern which is an image of the information used to drive the character generator. Generally, the light source is caused to scan the photoreceptor over successive scan lines until an entire image is produced. The image is developed and fused to print (image) receiving material.
In machines which utilize plain bond image receiving material or other ordinary material not specially coated, the electrophotographic process is of the transfer type where photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. The photoreceptive surface is moved to an imaging station where it receives light rays to produce the desired image. In the production of a positive image, the photoreceptive material is discharged to a relatively low level in white areas while dark areas continue to contain high voltage levels. In that manner, the photoreceptive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. desired to be reproduced. In the production of a negative image, discharge occurs in the dark portions.
After producing an image on the photoreceptor, the image is sent to a developing station where developing material called toner is placed on the image. This material may be in the form of a lack powder or liquid which carries a charge opposite in polarity to the charge pattern on the photoreceptor in a system which produces a positive image. In a negative image system, the toner would carry the same polarity as the charge on the photoconductor. Because of the attraction of the charged toner, it adheres to the surface of the photoreceptor in proportions related to the shading of the image. Thus, black character printing should receive heavy toner deposits, white background areas should receive none and gray or otherwise shaded half-tone character portions should receive intermediate amounts.
The developed image is moved from the developer to a transfer station where image receiving material, usually paper, is juxtaposed to the developed image on the photoreceptor. A charge is placed on the back side of the paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor. The remaining process steps call for permanently bonding the transferred toner material to the paper and cleaning residual toner left on the photoreceptor after the transfer operation. Thereafter, the photoconductor is reused for subsequent print production.
A common variation on the above-described process used in many electrophotographic machines involves the use of specially prepared material where the material itself carries a coating of photosensitive material. By utilizing that technique, the image is electrostatically placed directly on the image receiving paper. The paper is sent through a developer and then to a fuser for permanent bonding. Machines of this type avoid the residual toner problem and therefore avoid the need for cleaning stations. However, the resulting paper with its special photosensitive coating is more expensive than plain bond paper and special coating is considered to detract from the resulting product. As a consequence, coated paper machines are usually favored only for low volume applications or where quality product is not essential.
Regardless of whether a transfer process is used or whether specially coated paper is used, prior art machines have found it necessary to provide accurate paper placement in order that the image is placed on the paper at the proper position. In most machines, this has meant the provision of reference edges against which the paper is driven so that the side edge of the print receiving material is in a known, consistently-maintained position. Mechanical gates or gating rollers are frequently used to position the leading edge of the paper so that it mates with the leading edge of the image at a proper time. These prior art techniques have generally worked well but are the source of paper jams from time to time. One can appreciate that if the side edge of a paper must slip along a side reference edge without climbing over that edge and without moving away from that edge, a constraint is placed upon the paper feeding devices which occasionally results in malfunction. The inventors herein have devised a mechanism and a method for obtaining a proper image and image receiving material relationship without using side reference edges and thereby avoiding the jams which occasionally result therefrom. Additionally, by providing a proper image to paper relationship without reference edges, the speed at which the image receiving material can be moved to the image receiving position can be increased.